Preliminary Study of the Thermal Diffusivity of Source Sprayed Materials by the Flash Laser Method

Abstract

We have measured the thermal characteristics of plasma-sprayed coatings for thermal barriers. These coatings should be able to resist the thermal stresses caused by the presence of thermal gradients and the difference of thermal dilatation coefficients between the materials of the coating and of the substrate.

We have applied the plasma spraying process to multicoating systems and optimized the number and thicknesses of the coatings in order to reduce or eliminate the stresses. At this time approximately 60% of the thermal barriers are realized by the plasma spraying of zirconia with different stabilisers. We would like to find the thermal characteristics of the thermal barriers by measuring the diffusivity, in order to optimize the plasma spraying parameters.

We measure the thermal diffusivity by the flash method, from ambient temperature up to 1600 K and a pressure of 10-6 Torr. The flash is sent by a neodymium-glass Laser (energy pulse up to 100 J and pulse duration of about 450 µs) The samples have a diameter of about 1 cm and thickness up to several millimeters. The signal is detected by an I.R. detector and after amplification is sent to a digital oscilloscope which transfers the data to a minicomputer for calculating the diffusivity. For testing the apparatus we have measured copper samples having good accordance with known diffusivity values.

where T is the temperature of the sample at the time t and position L; Q is the density of the energy given by the flash; ρ, C and α are respectively: density, specific heat and thermal diffusivity. This result was obtained by the supposition that the sample is adiabatic and that the energy of the flash is uniformly deposited over the surface of the sample. We would like to take into account that the sample is not adiabatic and also the spatial distribution of the flash. For this reason we have measured the emissivity of the materials.

For zirconia we have also made a study of the transparency of this material for small thickness and high temperature. We have done a preliminary study of the influence of the transparency on measurements of the temperature of the back face of the sample.